1. Field of the Invention
This invention relates to a rubber composition for use in the manufacture of automotive tires.
2. Prior Art
Generally, tires particularly for their bead fillers have been formed from rubber compositions having a relatively high hardness parameter. There have heretofore been used large quantities of blends such as carbonblack, sulfur, thermosetting resins and the like for the purpose of providing increased dynamic modulus of elasticity (hereinafter referred to as elastic modulus) and hence enhanced hardness of the rubber composition. The use of large amounts of carbonblack is conducive to high hardness rubber compositions, but often results in a rubber composition which is difficult to integrate itself or wrap properly around the roll, not to speak of undue increases in the load on the Banbury mixer used.
Rubber compositions containing large quantities of sulfur are liable to undergo sulfur blooming during extrusion, leading to difficult tire forming operation and reduced product yields.
In an effort to eliminate or alleviate the above difficulties inherent in the use of large quantities of sulfur and carbonblack; that is, to hold their use to a practicable level, it has recently been proposed to use certain thermosetting resins such as novolak type phenol or modified phenol resins. The use of such resins in combination with carbonblack contributes to better processability of the rubber composition thanks to the thermosetting resin softening with the heat generated during the mixing operation, but it still aggravates the tire forming process due to unvulcanized rubber growing too stiff at room temperature, which in turn means a loss of the objectionably plied or bent extrudates.